The present invention provides a device and an image adjustment method by simultaneously adjusting the luminance and the chrominance of an image. The device comprises a luminance analysis device, a contrast adjusting device, a luminance adjusting device, a chrominance compensation device and a luminance gain adjusting device. The method comprises steps of: generating a maximum input luminance signal, an average input luminance signal and a minimum input luminance signal according to an input luminance signal; generating a contrast value according to at least one of the maximum input luminance signal and the average input luminance signal; generating an input luminance offset according to the input luminance signal and the minimum input luminance signal; generating an output chrominance signal according to the contrast value and an input chrominance signal; and generating an output luminance signal according to the input luminance offset and the contrast value.